This invention relates generally to wireless paging systems and more particularly to a system that effectively varies transmission coverage in different regions by varying the transmitted data rate.
Various issued patents and pending applications including U.S. Pat. Nos. 4,713,808 and 4,897,835 (both by Gaskill), U.S. PAt. No. 5,187,470 (King) and pending application Ser. No. 08/046,112, filed Apr. 9, 1993 show systems where digital data is transmitted on an FM subcarrier and modulated on a FM baseband. The data is transmitted to a pager receiver that filters the subcarrier from the rest of the FM baseband and then decodes the data on the subcarrier into digitally encoded messages.
FIG. 1 is a diagram showing two different geographical regions identified as region 12 (city A) and region 16 (city B). FM transmitters 14 in both region 12 and region 16 send digitally encoded messages on an FM baseband to portable pagers. The transmitters 14 in region 12 are spaced relatively close together in relation to the transmitters in region 16 and therefore, provide denser transmission coverage than the transmitters in region 16.
To explain further, each transmitter 14 in regions 12 and 16 has an associated transmission range 18. The transmission range is based on a given transmission data rate, transmission power, transmitter height, geographic surroundings, etc. The letters "R" represent receivers (e.g., pager receivers) positioned at different locations within regions 12 and 16.
When receiver "R" is located within range 18 of a transmitter 14, messages can be successfully received from the transmitter. In other words, within range 18 the strength of the transmission signal is sufficient for the receiver to successfully receive messages.
The transmission range for transmitters 14 extend over substantially the entire area in region 12 (i.e., dense transmission coverage). Thus, the receiver "R" can successfully receive paging messages from almost any location in region 12.
The transmission range for transmitters 14 in region 16 extend over only a small portion of the region (i.e., sparse transmission coverage). Thus, there are many locations in region 16 where receiver "R" will not be able to successfully receive messages. For example, no transmitter in region 16 has a range 18 that extends to locations 20, 22 or 24. Thus, receiver "R" cannot successfully receive messages in these locations.
As stated above, the transmission range 18 can be increased by increasing broadcast power. However, digital messages, as described above, are often transmitted on a subcarrier of an FM broadcast channel. Because, the broadcast power for FM stations is often restricted, it is not always possible to increase transmission coverage by increasing the transmitter output power.
Accordingly, a need remains for increasing the effective transmission range for transmitters in regions with sparce transmission coverage.